In known metered dose inhalers, the aerosol stream from a pressurised dispensing container is fired towards a patient or user of the inhaler into an air flow travelling in the same direction. In known devices, a user inhales through a mouth piece of the inhaler and creates an air flow through the container from air inlet holes which are generally at a part of the inhaler well spaced from the mouth piece. The medicament is then released into this air flow at a point between the air inlet holes and the mouth piece so that it is travelling in the same direction as the air flow. Typically in such devices, there is no restriction in the air flow between the air inlet holes and the mouth piece. Because of this, a substantial air flow may be created by a user of the device and, because the medicament is fired into the air flow in the same direction as the air flow, the effect is that particles of medicament can attain quite substantial velocities. As inhalers of this type are normally designed to be as small as practical for the convenience of users, the distance between the point at which the medicament is fired into the air flow and the patients mouth is usually quite small so that there is little distance to reduce the inertia of the particles of medicament with the result that the particles may impact in the oro-pharynx of a user with quite high velocity. This can be a problem with some medicaments.
In an effort to overcome this problem, devices have been produced in which the medicament is fired into a holding volume which allows the velocity of the medicament to be reduced and also allows some evaporation to occur.
U.S. Pat. No. 5,435,297 discloses a medical device for inhaling a metered aerosol comprising a cylindrical housing with a receiving chamber and first main air channels extending axially within the housing. A mouthpiece is co-axially connected to the housing. An atomising and vortexing chamber is delimited by the housing and the mouthpiece. Second main air channels extend within the mouthpiece and communicate with the first main air channels. Branch air channels extend at a slant to an axial direction within a partition positioned between the receiving chamber and the atomising and vortexing chamber. The branch air channels are connected to the first main air channels and open into the atomising and vortexing chamber. Inhaled airflow through the branch channels aids the atomisation of the aerosol. However, the branch channels do not slow the velocity of the dispensed medicament. Rather, they tend to increase the velocity of the medicament particles.
Devices such as those disclosed in U.S. Pat. No. 5,435,297, with a holding volume tend to be of significantly larger size than the standard metered dose inhalers and therefore less convenient and attractive to users.
One solution proposed in GB-A-2279879 uses a reverse flow. In this inhaler the air inlets are provided at a location axially between the air outlet of the duct means connecting an outlet of the container with the mouth piece and the mouth piece, and a passage is provided connecting the inlets to a location adjacent the outlet of the duct means. Thus when a user inhales through the mouth piece, an air flow is created from the inlet means to the mouth piece, the air flow having a component directed away from the mouth piece towards the outlet of the duct means.